Adds dynamic methods to scope.

=== Examples scope_posts_to_user { yield } #=> Post.with_scope({ :find => { :conditions => [ ‘user_id = ?’, params[:user_id] ]}, :create => { :user_id => params[:user_id] } }) { yield }

scope_messages_to_mailbox { yield } #=>
  Message.with_scope({
    :find => { :conditions => [ 'mailbox_id = ?', params[:mailbox_id] ]},
    :create => { :mailbox_id => params[:mailbox_id] }
  }) { yield }